The advantageous mechanical properties, dimensional stability and optical properties of polyester films are well-known and are exploited in many areas of technology. Many adaptations of polyester films have been proposed in order to tailor the properties of a polyester film to a particular application. However, an adaptation which improves the performance of a polyester film in one respect may be detrimental to the performance of the film in another respect. For instance, a modification to improve the optical properties of a film may have a detrimental effect on the mechanical properties of the film. Accordingly, it is often difficult to obtain polyester films having a suitable combination of desirable properties.
In order to improve the performance of polyester films, it is known to provide multi-layer films having complementary properties so as to form a composite structure. In some cases, multi-layer films may comprise two or more films of the same type in order to improve the mechanical strength of the film or to intensify other functional properties of the film. Alternatively, multi-layer films may comprise two or more films of different types, thereby enabling the properties of the different polyester films to be realised simultaneously. For example, multi-layer polyester films have been used to improve the handling properties of functional polyester films by disposing one or more films having desirable functional properties onto a base film having desirable mechanical properties. Laminated polyester films may suitably be prepared by co-extrusion, coating or lamination techniques.
The mechanical properties, dimensional stability, flexibility, weight, impact resistance and optical properties of polyester films offer advantages for their use in the manufacture of electronic or opto-electronic devices, such as electroluminescent (EL) display devices (particularly organic light emitting display (OLED) devices), electrophoretic displays (e-paper), photovoltaic (PV) cells and semiconductor devices (such as organic field effect transistors, thin film transistors and integrated circuits generally). The use of flexible polyester film as layer(s) in electronic devices allows the manufacture of such devices in a reel-to-reel process, thereby reducing cost.
A photovoltaic cell generally comprises a front-plane (or front-sheet); a front-side encapsulant material; the photoactive material on an electrode support substrate; a rear-side encapsulant; a rear back-plane (or back-sheet); and various components to collect and manage the electrical charge. Polyester films have been proposed in the manufacture of various layers in PV cells, for instance the front-plane, the back-plane, the electrode support layer(s). Photovoltaic modules, often consisting of many photovoltaic cells, are usually categorized according to the active photovoltaic materials used. These include crystalline silicon, gallium-arsenide (GaAs), amorphous silicon (a-Si), cadmium-telluride (CdTe), copper-indium-gallium-(di)selenide (CIGS), dye-sensitized or organic cells. Photovoltaic cells containing gallium-arsenide, amorphous silicon, cadmium-telluride, copper-indium-gallium-(di)selenide, dye-sensitized or conductive organic material are often referred to as thin-film photovoltaic cells (TFPV cells), which may or may not be flexible. Dye-sensitised PV cells are of particular interest, in which the active light-absorbing layer comprises a dye which is excited by absorbing incident light. Other thin-film silicon PV cells include protocrystalline, nanocrystalline (nc-Si or nc-Si:H) and black silicon PV cells. Thin-film photovoltaic cells are made by depositing one or more thin layers of photovoltaic material on a substrate, the thickness range of a thin layer varying from 1 or 2 nanometers to tens of micrometers, using a variety of deposition methods and a variety of substrates.
The back-plane, in particular, must exhibit good thermal dimensional stability. This has typically been a significant problem for polymeric materials, which tend to exhibit poorer dimensional stability than optical-quality glass or quartz. In PV cells generally, poor dimensional stability of a polymeric layer can result in the cracking of the adjacent encapsulant material, and particularly during the elevated temperatures (typically 130 to 160° C.; typically for up to 30 minutes) and normally also low pressure experienced during manufacture of the device. For instance, prior art films have been observed to exhibit wrinkling and movement during the manufacture of a PV device.
The back-plane should also exhibit good UV-stability. Lack of UV-stability can manifest itself in a yellowing, hazing and cracking of the film on exposure to sunlight thereby decreasing the effective service lifetime of the PV cell.
The encapsulant material is a barrier material which protects the photoactive and electrode layers and provides high resistance to gas and solvent permeation. The encapsulating barrier material is typically utilised in the form of a self-supporting film or sheet, which is applied to the composite comprising photoactive and electrode layers using lamination techniques, typically under vacuum, as is known in the art. The encapsulated composite is then sandwiched between a front-plane and a back-plane.
A problem with prior art devices has been the need for one or more additional primer layer(s) or surface treatment(s) to improve adhesion between the back-plane and the encapsulant. In particular, it has been necessary to coat the back-plane with a first adhesive inter-layer and then a second adhesive layer, wherein the second adhesive layer is a material having high adhesion to the encapsulant of the PV cell or is the same material as the encapsulant. It would be desirable to dispense with both of these additional adhesive layers and the additional process steps required to apply them, in order to increase manufacturing efficiency and reduce costs. In addition, it would be desirable to improve the adhesion between the back-plane and the encapsulant, relative to the prior art devices.
The back-plane should also exhibit good hydrolysis resistance. Poor hydrolytic stability can manifest itself by a reduction in mechanical properties and by the cracking of the film on exposure to moisture or other environmental conditions, particularly under humid conditions and/or elevated temperatures, and particularly on prolonged exposure over an extended period of time. The use of hydrolysis stabilisers in polyester film is well-known and these additives can play an important role in increasing hydrolysis resistance, but the introduction of such additives into polyester film can also have its disadvantages in film production and film quality. For instance, as hydrolysis stabiliser is introduced in increasing amounts, the intrinsic viscosity of the film can be reduced, the film can become discoloured and brittle and exhibit imperfections, and film formation can become difficult.